Imaging infrared optical systems of a cassegrain or catadioptric form for use in gun-launched applications benefit from low mass components. However, the typical material for the minors and for the structure of the assemblies of these optical systems is aluminum, which requires expensive machining to reach adequate accuracies. For example, current implementations of gun-launched imaging infrared optics are assembled from conventionally machined aluminum optical components which are both labor and equipment intensive, thereby increasing cost. Resulting designs are not lightweight enough, nor inexpensive enough, and therefore do not meet the low cost and low mass requirements for such imaging systems.
Previous design attempts reduce mass and cost by molding fiber-reinforced polymer mirrors, but the resulting surface finish is not adequate for use as a minor surface due to the presence of the reinforcing fibers. On the other hand, design attempts with unreinforced polymers have adequate surface finish, but do not have adequate strength for use in gun-launched systems. For example, Ultem® is an amorphous thermoplastic polyetherimide (PEI) material manufactured by SABIC Innovative Plastics, and used as a polymer mirror material. Previous attempts also suffer performance loss from distortion of the optics at high temperature. Optical distortion at high temperature is due to the difference in coefficients of thermal expansion (CTE) of multiple materials used in the optical system design, as well as due to the difference in CTE between the materials used in the optical system design and the material of an external platform to which the optical system mounts.
For example, mirrors molded from a milled glass-filled Ultem® material are stiff enough to survive launch shock and close enough in CTE to an external aluminum gimbal platform such that thermal distortion would be fairly minimal. However, surface roughness characterization results reveal that as-molded, filled Ultem® polymers are extremely poor candidates for mirror substrates, as their surface roughness is far too high to yield a precision optic. On the other hand, surface roughness characterization reveals that unfilled Ultem® is a fine optical substrate material. However, without the filler, the polymer has a much larger CTE as well as lower stiffness and strength. Therefore, mounting distortion over temperature would result in drastically reduced minor performance, and stress levels during launch shock could induce structural failure.